Titanium dioxide is widely used as a white pigment in paints, plastics and paper. Two types of titanium dioxide are produced, namely rutile and anatase.
There are two commercial processes for the production of titanium dioxide pigment, namely the sulfate process and the chloride process. The sulfate process is capable of using a feedstock with a relatively low titanium dioxide content, such as ilmenite. However, the capital costs of a modern sulfate process can be higher than that of an equivalent chloride process plant. Furthermore, there is a higher volume of waste products to be treated and disposed of in the sulfate process due to the use of a more impure feedstock and the fact that the sulfuric acid used in the process cannot be readily recovered and recycled.
Accordingly, the chloride process is now a more common process and is growing relatively more rapidly than the sulfate process. However, the feedstock suitable for use in the chloride process usually needs to have a higher titanium dioxide content and to contain fewer impurities than those which are suitable for the sulfate process.
Titanium dioxide is commonly found in nature in the form of ilmenite (FeO.TiO2) which may contain from 40% to 80% titanium dioxide, higher grade feedstock also being referred to as leucoxene. Titanium dioxide is also found in nature as a mineral rutile, in which form it has been extensively mined as a feedstock for the chloride process. Deposits of other titanium bearing feedstock have been discovered but because of the grade or technical difficulties have not yet been commercially exploited.
Naturally occurring ilmenite is sometimes used in the chloride process but its low titanium dioxide content coupled with the need to dispose of excessive quantities of chloride waste has meant that its use has been limited.
Processes have been developed with the aim of upgrading the titanium dioxide content of ilmenite to a value approaching the equivalent of natural rutile. These processes involve treatment of the ilmenite to remove the iron fraction from the ilmenite by leaching. The first stage of such treatment is the potentiation of the ilmenite to make it more susceptible to leaching. Such potentiation treatments include the partial reduction of the iron fraction in the ilmenite; pre-oxidation of the ilmenite feed followed by partial reduction of the iron fraction in the ilmenite; reduction of the iron fraction to the metallic state, such as the Becher process or smelting of the feedstock. “Potentiation” of the feedstock is understood to create pores, passages, voids or similar spaces in the body of the material as well as convert the iron component to a more readily leachable oxidation state whereby chemical action by acid is enhanced.
Once the feedstock has been prepared leaching is then carried out. Several leaching procedures are known in the prior art among which are the following:                1. Leaching in a batchwise process in rotating spherical pressure vessels at an elevated temperature to enhance the reaction rate; one such process is known as the Benilite process;        2. Leaching in vertical cylindrical liquid/solid fluidised bed reactors; one such process is the Laporte process;        3. Leaching in staged vertical fluidised cylinders, in such a manner that the leach liquid flows continuously from one stage to another in a counter current fashion whilst the solids are fed and removed in a batchwise manner in a single stage cylinder; one such process is the Tiomin Synthetic Rutile (TSR) process.        